In a past OMNIview, we featured an article on how we used stereolithography parts (click to read the article) to construct complex appearance models. Another use for those accurate (but not very robust) plastic parts is for producing original patterns when building silicone-rubber molds, also called "soft tools". We use soft tooling for small parts and larger desktop-sized cases when we need multiples of less than twenty or thirty.
The advantages of silicone molds over other tooling methods are low cost, speed, versatility, and accuracy.
Costs for the molding materials and the stereolithography model/ pattern are usually measured in hundreds, rather than thousands of dollars. After the molds are built, duplicates can be cast in a matter of days. The flexible nature of a soft tool also allows the engineer to test different casting materials without changing drafts. Best of all, the resultant prototype parts are accurate within a few thousandths of an inch.
We build molds that range from pea-sized to some that are as large as a toaster oven.
First a mold container (MDF or acrylic) is constructed, then the original part is suspended within. See picture at left. (The yellow clay was inserted so that the original part would not be completely encased in the cured silicone mold. Before the top half of the mold is poured, the clay will be removed. This way, the two mold halves can be separated after they're cured.)
Liquid silicone is poured into the container, covering and completely encasing the original part. See picture at right. In a few hours, the silicone cures (this process can be accelerated with heat) and the hardened rubber block is removed from the mold box.
The next challenge is to extract the original encased part without damaging either it or the new "soft tool". An experienced and steady hand is needed to cut cured silicone in the right places so that the original model can be safely removed. The resulting flexible mold can be bent open to remove the original part. See picture at left which shows half of a two-piece mold. If the part is a complicated one, it may need to be sectioned.
After re-assembly, the void where the original stereolithography part was originally located is filled (cast) with the material of choice, usually a two-component epoxy or urethane. The fill material is cured, and the new cast replica is removed from the soft tool. Depending on the material used, we can make a score of duplicates from one mold. Picture at left shows both mold halves with an actual cast part after it was removed. A close look at the picture reveals "sprues" (they look like stalagmites) on the cast part. They are the result of holes that were added to the mold to inhibit bubble formation in the finished part. The sprues are removed during final finishing.
We used this prototyping process for the Hampton lock housing project (see OMNIview 17); one that required a lot of testing with snap-fits, crush ribs, and material selection. The accuracy of the resultant parts and the ability to test different production materials significantly impacted development time. It was the “soft-tooling” that made it possible for us to meet our customer's time line.
Read about our full time model maker, Mike Ammerman, and his amazing modelmaking abilities.